1. Field of the Invention
The present invention generally relates to a power conversion apparatus, and more particularly, to a power conversion apparatus with a leading edge blanking (LEB) function for preventing an over current protection (OCP) mechanism from being activated/triggered incorrectly.
2. Description of Related Art
A power conversion apparatus is capable of converting the high and instable AC input voltage supplied by an electric utility into a low and stable DC output voltage suitable for an electronic device. Thus, power conversion apparatuses have been broadly applied to different electronic devices, such as computers, office automation equipments, industrial control equipments, and communication equipments.
FIG. 1 is a diagram of a conventional power conversion apparatus 10. Referring to FIG. 1, the pulse width modulation (PWM) signal generator 101 outputs a PWM signal VPWM to switch (i.e., turn on and off) the power switch Q. Accordingly, the AC voltage (determined by the turn ratio between the primary side P and the secondary side S of the transformer T) at the secondary side S of the transformer T corresponding to the AC input voltage Vin received by the primary side P of the transformer T is converted into a DC output voltage Vout through rectification of the diode D and filtering of the capacitor C and supplied to the electronic device LD.
However, if the current Ip flowing through the primary side P of the transformer T is too large (i.e., an over current is produced), the power switch Q and/or the electronic device LD may be damaged. Conventionally, an over current protection (OCP) unit 103 is disposed in the power conversion apparatus 10 for detecting the voltage Vcs (Vcs=Rs×Ip) on the node N1 between the power switch Q and the resistor Rs. When the voltage Vcs on the node N1 reaches a predetermined OCP reference voltage of the OCP unit 103, the OCP unit 103 activates an OCP mechanism to stop the PWM signal generator 101 from outputting the PWM signal VPWM and turn off the power switch Q, so that the power switch Q and/or the electronic device LD can be protected.
However, a spike may be produced at the instant when the power switch Q is turned on because of (in response to) the PWM signal VPWM, and as a result, the voltage Vcs on the node N1 detected by the OCP unit 103 may be very high (or may even reach the predetermined OCP reference voltage of the OCP unit 103) at that instant. Thus, the OCP unit 103 may activate/trigger the OCP mechanism incorrectly. In this case, the power switch Q is turned off when no over current is produced in the power conversion apparatus 10, and accordingly, the power conversion apparatus 10 cannot supply the DC output voltage Vout to the electronic device LD as expected.